The in-situ delivery of therapeutic agents within the body of a patient is common in the practice of modern medicine. In-situ delivery of therapeutic agents is often implemented using medical devices that may be temporarily or permanently placed at a target site within the body. These medical devices can be maintained, as required, at their target sites for short or prolonged periods of time, in order to deliver therapeutic agents to the target site.
For example, in recent years, drug eluting coronary stents, which are commercially available from Boston Scientific Corp. (TAXUS), Johnson & Johnson (CYPHER) and others, have become the standard of care for maintaining vessel patency after balloon angioplasty. These existing products are based on metallic balloon expandable stents with biostable polymer coatings, which release antiproliferative drugs at a controlled rate and total dose.
Nanoporous materials have the potential to revolutionize drug delivery. For example, iMEDD, Inc. has created silicon membranes with parallel channels ranging from 4 to 50 nm. Diffusion rates of various solutes through such membranes have been measured and conform to zero-order kinetics in some instances (i.e., release is constant with time). This is in contrast with typical situations in which drug diffusion rates decay with time, because the concentration gradient, and thus the driving force for diffusion, is also decaying with time. Diffusion is ordinarily governed by Fick's law, which states that the flux of a given substance (i.e., the amount of the substance crossing a unit area per unit time) arising from molecular diffusion is directly proportional to the concentration gradient of the substance. One explanation for zero order behavior is that, by making the diameter of the nanopores only slightly larger than that of the drug, the nanopores act as bottlenecks, forcing the drugs to proceed in a substantially single-file fashion through the membrane. iMedd claims that the membranes can be engineered to control rates of diffusion by adjusting channel width in relation to the size of solutes. When the proper balance is struck, zero-order diffusion kinetics is possible. iMedd has produced a drug delivery device which consists of a drug-filled enclosure which is fitted with a nanoporous membrane as the only connection between the internal reservoir of the device and the external medium.